In plain words
Every small word too arose and spread abroad in the form of a mantra. Satyaṃ vada, dharmaṃ cara, Om Brahma, satyaṃ jñāna manantaṃ brahma, ekaṃsat viprābahudhā vadanti, eko'haṃ bahusyāṃ, eko vaśī sarvabhūtāntarātma: all these, through small words alone, took on the form of mantras, spread through the world, taught man and gave him Ānanda. Another wonderful secret in this Upaniṣad: each Upaniṣad was fitted only to people of one level; this Taittirīya Upaniṣad taught brahmacārīs, householders, vānaprasthas and sannyāsīs, all alike and evenly, and took on a form every human being needs. Every teaching this Upaniṣad gave is most needed by those in all four āśramas.
There is another passage in it: it taught about the pañcāgnis. These are five kinds of fire. Udarāgni, mandāgni, kāmāgni, śokāgni, baḍabāgni. The first is udarāgni. The word is big, but its meaning is very small. What burns with hunger they called udarāgni. Mandāgni: what burns with indigestion after eating beyond measure they called mandāgni. Kāmāgni: the citta that burns with many kinds of desires and cravings is itself what has been called kāmāgni. Śokāgni. Śokāgni is not grieving for worldly cares and worldly cravings. All the burning he feels as he longs for the vision of God, for the Ānanda of God, for reaching God, has become śokāgni. The Bhagavad Gītā is a book the whole world and people of every faith have accepted. Right where it opens, the very first chapter they called Viṣāda Yoga. What is this viṣāda? Is he enquiring into a matter of the world alone? Is he grieving for his kin and friends? Why did grief of this kind receive the fitting name of yoga? So this is not a grief grieved for physical and worldly pleasure and pain and happiness, or for enjoyments and riches, or for wealth and gold, goods and vehicles. For God's sake and for Dharma's sake he fell into grief. Through war many lives perish. With these men perishing, Dharma declines.